Five days after the country's inconclusive election, which produced no outright winner, the third-placed Liberal Democrats readied themselves to side with Cameron and send his Conservative Party back into power for the first time since 1997.
Talks between Nick Clegg's Liberal Democrats and Prime Minister Gordon Brown's incumbent Labour Party quickly unraveled Tuesday amid doubts on both sides, despite Brown's offer Mondayin order to seal a pact.
Labour lawmakers downplayed the chances of their party clinging to power, saying that they lacked the mandate to govern after finishing a distant second in last week's election to the Conservatives.
"We have got to respect the result of the general election and you cannot get away from the fact that Labour didn't win," Labour's Health Secretary Andy Burnham told the BBC.
"The key thing is Gordon Brown knows the game is up," Conservative Party lawmaker Michael Fabricant said, ahead of meetings later Tuesday expected to confirm a pact between the Liberal Democrats and the Conservatives.
Liberal Democrat deputy leader Vince Cable told Sky News a deal was "very close to being done."
Conservative negotiator William Hague, likely to be appointed Foreign Secretary, said a deal between his party and Clegg's group was the only credible outcome.
"There should be a government with a strong and secure majority in the House of Commons," Hague said.
If a deal for Clegg and Cameron to join in a formal coalition, or possibly a looser alliance, is sealed later Tuesday, the Conservative chief is likely to travel to Buckingham Palace on Wednesday to be confirmed as the country's new leader by Queen Elizabeth II.
Brown's office declined to say whether he planned to tender his government's resignation imminently. A Downing Street spokesman said only that Brown was likely to make a statement if any deal between Clegg and Cameron is confirmed.
Aides also refused to discuss reports that Brown, 59, will quit politics altogether - triggering a special election by resigning his House of Commons seat.
Some Labour lawmakers said they feared a pact between Labour and the Liberal Democrats - dubbed a "coalition of the defeated" by Conservatives - would lack legitimacy and prompt angry reactions from voters.
Public anger was already visible. Crowds grew outside Parliament on Tuesday with some people shouting at passing lawmakers, "Make up your mind!" Others unfurled banners calling for the political system to be overhauled.
Former Labour Party communications director Lance Price told ITV News that Labour would go into opposition, and that senior figures should step aside. "It will be time for a new generation of politicians to take over," he said.
Clegg's Liberal Democrats are reported to have won concessions from Cameron on their key demand - that Britain change its voting system toward a more proportional system.
Although Cameron's party bitterly opposes changing the current voting system - which favors Britain's two main parties, Conservatives and Labour - negotiators have pledged to "go the extra mile" to strike an agreement with Clegg.
Cameron's team said they offered a public referendum on a more proportional system - but confirmed they will campaign still against any changes. They said Liberal Democrat lawmakers would be offered Cabinet posts, meaning Clegg could even be appointed as deputy prime minister.
Clegg wants a European-style proportional representation voting system, which he believes could greatly increase his party's future seats in Parliament. In the latest election, Liberal Democrats won almost a quarter of the overall vote but only 9 percent of House of Commons seats.
Most European countries use proportional-representation rules for elections, allowing parties that win 10 percent of the vote to get about 10 percent of the parliamentary seats. The system makes coalition governments common, but it also involves longer, more complex votes with several rounds.
Close results are much more common with proportional representation than in the single-winner approach observed in Britain, whose system now tends to yield large majorities in Parliament.
The Conservatives won 306 of the 650 seats in the House of Commons last week - just short of the 326 needed for a majority. Labour won 258 seats, Liberal Democrats won 57, and smaller parties took the rest.
Liberal Democrat lawmaker Roger Roberts said a decision on the Conservative offer would be made at a party meeting later Tuesday, and it was not certain the Liberal Democrats would accept.
"If necessary, we go into this alliance, but we must get electoral reform and a good package for the economy," he told the BBC.
Cameron's office declined to say whether they expected a deal to be announced later Tuesday, and Buckingham Palace would not comment on whether the queen was expecting a visit from the prime minister.
"I'm just in the dark, like all of you," Cameron said, as he left a meeting with new Conservative lawmakers at the House of Commons.
Financial markets kept a close watch on developments. The British pound, which fell earlier in the day as talks dragged on, was up more than one U.S. cent at $1.4983 in afternoon trade as signs emerged of a Conservative breakthrough. The London Stock Exchange's leading FTSE 100 index, which closed before the news, ended the day down 2.6 percent at 5,334.21.